1. Field of the Invention
The present invention relates to a pigment dispersed aqueous ink composition containing a certain polysaccharide compound as a pigment dispersant.
2. Background Art
Ink jet printing is a non-impact printing process in which the printer produces droplets of ink in response to digital signals, such as those generated by a computer. The droplets of ink are deposited on a substrate such as paper or transparent film. Ink jet printers have found broad commercial acceptance due to their print quality, low cost, relatively quiet operation, and graphics capability. Thermal (bubble jet) and piezoelectric drop-on-demand printers have been especially successful in the marketplace and have found broad application as printers for personal computers in the office and the home.
The inks used in ink jet printers can be classified as either dye based inks or pigment based inks. Dye based inks are satisfactory for many applications but have poor light fastness and poor water resistance. As a printed document is expected to have a certain degree of permanency, the lack of light fastness and water resistance of the printed image derived from dye based inks is a significant problem. Pigment based inks can be prepared which have excellent light fastness and water resistance and are preferred over dye based inks for generating printed documents with a reasonable degree of permanency.
In spite of the permanency advantage of pigments over dyes, the introduction of pigment-based inks into the consumer ink jet printer market has been very limited. One reason for this is that formulating pigment based inks for use in ink jet printing which give reliable printing performance and yield rapidly drying printed images with levels of color bleed, which are equal to or better than those obtained from comparable dye based inks has proven to be very difficult.
Some dye based inks, for use in ink jet printing, which give reliable printing performance and are rapidly drying, are already formulated and typically contain a water as the principal solvent, at least one humectant cosolvent, at least one penetrating cosolvent, at least one surfactant, and at least one dye. The combination of the penetrating cosolvent(s) and the surfactant(s) results in an ink composition which has a surface tension in the range of 30 to 40 dyne/cm and which penetrates porous media, such as paper, very rapidly. These low surface tension penetrating inks give high area coverage per drop. High per drop coverage is economically desirable since less ink is used per page. The fast penetrating ability of these dye based inks results in printed images which show negligible color bleed. Color bleed is defined herein as the mutual invasion of one color into another color which can occur when two different color inks are printed adjacently. Negligible color bleed results in a border between the two color regions which is clean and free from invasion of one color into the other.
The problem with formulating pigment based inks, which have low surface tensions and which penetrate porous media very rapidly, is that commonly used penetrating cosolvents and surfactants destabilize most pigment dispersions. Specifically, the penetrating cosolvents and surfactants cause flocculation of the pigment dispersion by displacing pigment dispersants from the surfaces of the pigments. Approaches to make pigment dispersants flocculation-resistant have tended to result in inks in which bubbles form easily. Although, such inks are printable at relatively slow speeds, they cannot be ejected reliably at acceptably fast print speeds in either thermal (bubble jet) or piezoelectric drop-on-demand printers.
Thus, there remains a need for a pigment dispersed aqueous ink composition for use in ink jet printing which can be formulated using a combination of penetrating cosolvents and surfactants, and yet, which gives reliable printing performance at acceptably fast print speeds. There remains a need for a pigment dispersed aqueous ink composition which can be formulated using a combination of penetrating cosolvents and surfactants such that rapidly drying printed images with levels of color bleed, which are equal to or better than those obtained from comparable dye based inks, can be obtained.
The inventor of the present invention has now found that a certain polysaccharide compound can be used as a pigment dispersant.
Accordingly, it is an object of the present invention to provide a pigment dispersed aqueous ink composition for use in ink jet printing which gives reliable printing performance and yields rapidly drying printed images with levels of color bleed, which are equal to or better than those obtained from comparable dye based inks.
According to the present invention, there is provided a pigment dispersed aqueous ink jet ink composition comprising water as the principal solvent, a water-soluble organic solvent, and a polysaccharide compound represented by the formula: 
wherein
A represents a group represented by the following formula (i) or (ii): 
x is the average value for the compound of the formula (I) in the ink composition and is in the range from 4 to 14,
n is in the range from 2 to 12,
m is either 2 or 3, and
R1 and R2 independently represent a C5-20 aryl group which may be substituted, an aralkyl group which may be substituted, and the heterocyclic ring containing at least one nitrogen atom, which ring may be substituted.